Vibrational Spectrum And Polarization Properties Of Imidazole Ionic Liquids On Graphene

As an environmentally friendly solvent,ionic liquids?ILs?have excellent properties such as low melting point,low vapor pressure,wide electrochemical window,recyclability,non-flammability and thermal or chemical stability makes ILs materials are promising candidates for a number of different energy applications.A particularly interesting example is in next generation supercapacitors,where ILs as electrolytes in combination with graphene is being explored as an electrode material.Since the amount of energy stored in the supercapacitor primarily depends on the electrode electrolyte interface,a thorough understanding of the graphene/ILs interface is required.The vibrational spectrum?VS?is a multi-functional tool to display the structural characteristics of solid and liquid materials,while the simulation method based on density functional theory?DFT?can study the VS of ionic liquids more accurately on the micro scale.Therefore,in this study,the VS of ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate?[Emim][BF₄]?and graphene-[Emim][BF₄]in the range of 10-3500cm^-1 were simulated and calculated by DFT method.For the first time,the influence of graphene on the vibration spectrum of[Emim][BF₄]was systematically analyzed based on DFT calculation.These vibrational peaks position changes mainly due to the spatial position of anion relative to cation is changed by the interaction what is dominated by dispersion interaction between graphene and[Emim][BF₄].The vibrational peaks of[Emim][BF₄]at 86.91,816.36 and 975.22 cm^-1 were changed mainly due to the reduction of the interaction between cation and anion of[Emim][BF₄]on the graphene surface(from-367.06 to-356.06 kJ·mol^-1).The vibrational peaks of[Emim][BF₄]at882.46,1145.49 and 3170.26cm-1 have significantly changed,which is mainly caused by the change of the hydrogen bond strength between ions.The vibrational peaks of[Emim][BF₄]at 101.49,126.40,1070.20,1336.15,1402.15,1417.43,1455.58,1457.92,2955.87,2963.32,3034.01,3043.80,and 3054.38 cm^-1 were all blue shifted due to the induction between anion and cation of[Emim][BF₄]absorbed on graphene surface was enhanced(from-44.68 to 45.42 kJ·mol^-1).And then,the influence mechanism of graphene on the dipole moment of[Emim][BF₄]was analyzed based on DFT calculation.The dipole moment represents the separation between the positive charge distribution and the negative charge distribution,that is,the overall polarity of the charge system.In the process of studying and analyzing the vibration spectra of the[Emim][BF₄]ion pair on graphene surface,it is found that graphene also have had a certain influence on the charge distribution of[Emim][BF₄].By DFT calculation,it was found that the dipole moment of the[Emim][BF₄]ion pair on graphene surface increased from 11.93 to 14.46 Debye.Next,the dipole moment of each atom was calculated based on the atomic in molecules?AIM?,and the changes of electron density distribution of some atoms with obvious changed were analyzed in detail.It is found that the electron density distribution of imidazole ring of[Emim][BF₄]on graphene whole toward near the surface of graphene and away from the direction of the anionic diffusion,and the electron density distribution on the anion toward close to the graphene surface and away from the direction of the cation diffusion,so the center distance of positive and negative increased and then caused the dipole moment changed direction and the value increased.And the AIM charge of[Emim][BF₄]is further analyzed,and the conclusion is consistent with the result of electron density analysis.Through observation the dipole moment direction of[Emim][BF4]on the graphene,guessed that the ionic liquids on graphene surface have formed a layer of ions in the layer thickness of regular arrangement structure exactly due to the change of dipole moment of ionic liquids on the graphene surface,the angle between the graphene-ILs dipole moment and the graphene plane is very small,which is the basis of the structural stability.The research of vibrational peak move of[Emim][BF₄]on graphene surface can better understanding the interaction between the graphene and ionic liquids,and further studied the mechanism of the effect of graphene on the dipole moment of[Emim][BF4],will great guiding significance for design and selection of the ionic liquids in industrial production.